Question 1

What is the minimum inhibitory concentration (MIC) and how is it determined?

Accepted Answer

The minimum inhibitory concentration (MIC) is the lowest antibiotic concentration at which bacteria can no longer survive or multiply. It is determined by observing where bacterial growth stops in response to increasing antibiotic concentrations. In an E-test, the MIC value is read at the point where the growth inhibition zone intersects the antibiotic strip. In broth dilution methods, MIC is identified as the lowest antibiotic concentration showing no visible bacterial growth or turbidity.

Question 2

How does the Epsilometer test (E-test) measure antibiotic susceptibility?

Accepted Answer

The E-test uses a plastic strip impregnated with a predefined gradient of antibiotic applied to a Mueller-Hinton agar plate inoculated with bacteria. The antibiotic diffuses into the agar, and susceptible bacteria cannot multiply, forming a clear growth inhibition zone around the strip. The MIC value is determined by reading the numerical scale on the strip at the point where bacterial growth intersects the strip edge.

Question 3

What is antibiotic synergy and how is it evaluated?

Accepted Answer

Antibiotic synergy occurs when the combined effect of two antibiotics is greater than the sum of their individual activities. Synergy is evaluated by calculating the fractional inhibitory concentration (FIC) index, which sums the ratio of each antibiotic's MIC in combination to its individual MIC. An FIC index less than 0.5 indicates significant synergy between the two antibiotics.

Question 4

What is the difference between cross and non-cross E-test methods for synergy testing?

Accepted Answer

In the non-cross test, E-strips are applied sequentially to the same plate location after the first strip is removed, requiring two plates and more time. In the cross test, both E-strips are placed simultaneously at a 90-degree angle on a single plate, making it faster and requiring fewer materials. Both methods determine MIC values in combination and calculate the FIC index to assess synergy.

Question 5

How does the broth micro dilution method determine MIC values?

Accepted Answer

The broth micro dilution method creates two-fold serial antibiotic dilutions in a 96-well microtiter plate containing bacteria in growth medium. After incubation, the MIC is identified as the well with the lowest antibiotic concentration showing no visible bacterial growth or turbidity. This quantitative method offers flexibility in choosing specific antibiotic concentrations and can be applied to multiple bacterial species.

Question 6

Why is preparing a standardized bacterial suspension important for antibiotic susceptibility testing?

Accepted Answer

A standardized bacterial suspension ensures consistent and comparable results across tests. The suspension is prepared by matching its turbidity to a McFarland 0.5 standard, which establishes a uniform bacterial concentration. This standardization is critical because varying bacterial concentrations would produce inconsistent growth inhibition zones and unreliable MIC values.

Question 7

What does an FIC index greater than 0.5 indicate about antibiotic combinations?

Accepted Answer

An FIC index greater than 0.5 indicates that the two antibiotics do not act synergistically against the tested bacterial strain. Instead, their combined effect is either additive, meaning the joint effect equals the sum of individual activities, or potentially antagonistic. This result suggests that combination therapy may not provide enhanced efficacy compared to single antibiotic treatment.